Referring to the last generation of CRT displays, a lot of work has been done to improve its picture quality. Consequently, a new technology like Plasma has to provide a picture quality at least as good or even better than standard CRT technology. For a TV consumer, high contrast is one main factor for a high subjective picture quality of a given display. The dark room contrast is defined as the ratio between the maximal luminance of the screen (peak-white) and the black level. Today, on plasma display panels (PDP), contrast values are inferior to those achieved for CRTs.
This limitation depends on two factors:                The brightness of the screen is limited by the panel efficacy that in general is lower than that of a CRT for a given power consumption. Nevertheless, the PDP efficacy has been constantly improved during the last years for the benefit of contrast.        The black level of the PDP screen is not completely dark like on a CRT. In fact, a backlight is emitted even while displaying no video signal. The plasma technology requires for the successful writing of a cell a kind of pre-excitation in the form of a regularly priming signal representing an overall pre-lighting of all plasma cells. This priming operation is responsible for the backlight, which drastically reduces the PDP contrast ratio. This reduction is mostly visible in a dark room environment representing the major situation for video applications (home theatre etc.)        
In the following, aspects of response fidelity and priming are presented in more detail.
A panel having good response fidelity ensures that only one pixel could be ON in the middle of a black screen and in addition, this panel has a good homogeneity. FIG. 1 illustrates a white page displayed on PDP having response fidelity problems. The response fidelity problems appear in the form of misfiring of cells having too much inertia. Such cells require more time for writing as available.
A first solution to achieve good response fidelity, by standard PDPs and for a given addressing speed, leads to the priming operation mentioned above. In that case, each cell will be repeatingly excited. Nevertheless, since an excitation of a cell is characterized by an emission of light, this has to be done parsimoniously to avoid a strong reduction of the dark room contrast (i.e. to avoid more background luminance). Therefore a simple way to improve the dark room contrast leads to an optimization of the priming use.
Actually, two kinds of priming can be found on the market:                “Hard-priming” which generates more backlight (e.g. 0.8 cd/m2) but which has a very high efficacy. Usually, one single “hard priming” per video frame is sufficient.        “Soft-priming” which generates less backlight (e.g. 0.1 cd/m2) than the previous one but has less efficacy. On many products, this priming is used for each sub-field, which leads to a very poor dark room contrast again.        
Obviously, the better solution should be based on the use of a “soft-priming” with the assumption that the total amount of “soft-priming” required to obtain an acceptable response fidelity will produce less light than a single “hard-priming”. This is not the case when the coding has not been optimized since one priming per sub-field should be required.
In fact, the best contrast ratio will be obtained by using a single soft-priming operation per frame. Such a concept is achieved by optimization of the coding concept as seen in the next paragraph.
The document EP-A-1 250 696 introduces a concept of one single “soft-priming”, where only one priming at the beginning of a frame is performed. In that case, only the first sub-fields will be near enough from the priming signal in the time domain to benefit from it. Now, the main idea was to use these first sub-fields as a kind of “artificial priming” for the next sub-fields taking the assumption that one lighted sub-field will help the writing of the next ones (cascade effect). FIG. 2 illustrates this “cascade effect” in the case of a 12 sub-fields code by analyzing the jitter of the writing discharge for the last sub-field (most significant bit MSB). It represents the statistic distribution of the writing discharge of the last sub-field inside the plasma cell for two different codewords by respective envelope curves. In both situations, there is only one priming (P) at the beginning of the frame (not shown).
In the first case, the codeword used (P-101111111101) enables a good cascade effect from the priming P up to the last sub-field (MSB). Then, the distribution of the writing discharge is well concentrated and fully occur inside 1.1 μs which represents the new borderline for the address speed. This means, that the writing process can be performed within the addressing period.
In the second case, the codeword used (P-000000000001) does not permit any cascade effect and therefore the writing of the last sub-field is less efficient. Then, the distribution of the writing discharge is no more concentrated and is spread on a longer time period as shown by the envelope. Thus some writing process would be performed after the addressing period. In that case, more time should be given to the addressing for acceptable response fidelity.
The results presented in FIG. 2 have shown that good response fidelity can be obtained through a kind of cascade effect from the priming up to the highest sub-field. In that case the initialization started with the priming will spread like a wild fire among the whole frame. Therefore, an optimized concept will require a concentration of energy around the low sub-fields, which are the most critical ones to ensure them a maximal benefit from the priming. In addition to that, the time delay between two consecutives lighted sub-fields should be kept as small as possible to increase the influence between them and to produce an optimal cascade effect starting with the priming.
FIG. 3 illustrates various ways to encode the video level 33 with two different sub-field organizations. Depending on the sub-fields organization, there are one or more encoding possibilities for a video value. A binary code shown on the left side of FIG. 3 leads to a large space between two sub-fields ON. Therefore, there is no influence between these sub-fields and no concentration of energy in the low sub-fields. As a result, more priming or longer addressing time is needed. A redundant code presented on the right side of FIG. 3 enables a better concentration of the energy around the priming and also enables to reduce the distance between two sub-fields ON so that the cascade effect can be utilized.
Moreover, the optimal sub-fields encoding should enable to have not more than one sub-field OFF between two sub-fields ON. This property will be called Single-O-Level (SOL). An optimized sub-field weighting based on the mathematical Fibonacci sequence enables to fully respect the SOL criterion.
FIG. 4 illustrates an example of coding used for all further explanations (11 sub-field redundant coding). The frame depicted here starts with a priming operation. After that, a sequence of sub-fields follows. Each sub-field starts with an addressing block. According to the value of the sub-field a time period for applying sustain impulses follows. At the end of each sub-field a plasma cell is reset by an erasing operation.
Nevertheless, some experiments have shown that, under some circumstances, even a SOL criterion combined with a single “soft-priming” is not enough to provide perfect response fidelity.
In the following the specific problem of the present invention is demonstrated. Experiments have shown that, when the number of sustains grows, the biggest sub-fields will suffer from response fidelity problems. These problems appear only under certain circumstances, for instance in the case of a horizontal greyscale at a high sustains number as shown in FIG. 5. When the number of sustains is increased, some response fidelity problems appear at the PDP borders. However, this does not appear in a homogeneous way but only some specific video levels are disturbed.